Schedule I-Condensed Financial Information of Registrant (Details) (USD $)	3 Months Ended											12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009		Dec. 31, 2008
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,065,749,000		$ 1,152,181,000	$ 930,757,000	$ 901,805,000	$ 1,042,551,000		$ 1,125,923,000		$ 906,444,000	$ 834,777,000	$ 4,050,492,000		$ 3,909,695,000		$ 4,176,501,000
Operating expenses	939,172,000		1,032,760,000	848,454,000	823,739,000	912,377,000		1,016,961,000	[1]	817,782,000	751,848,000	3,644,125,000		3,498,968,000		4,329,598,000
Operating income (loss)	126,577,000		119,421,000	82,303,000	78,066,000	130,174,000		108,962,000	[1]	88,662,000	82,929,000	406,367,000		410,727,000		(153,097,000)
Other income												6,520,000		8,018,000		9,331,000
Interest expense												81,354,000		83,011,000		84,099,000
Income taxes												110,274,000		122,530,000		(96,092,000)
Net income (loss)	61,008,000	[2]	63,974,000	45,067,000	42,977,000	88,939,000	[3]	61,010,000	[1]	48,938,000	41,772,000	213,026,000		240,659,000		(123,274,000)
Dividends on preferred stocks												685,000		685,000		685,000
Earnings on common stock												212,341,000		239,974,000		(123,959,000)
Current assets:
Cash and cash equivalents	162,772,000					222,074,000						162,772,000		222,074,000		175,114,000
Receivables, net	646,251,000					583,743,000						646,251,000		583,743,000
Inventories	274,205,000					252,897,000						274,205,000		252,897,000
Deferred income taxes	40,407,000					32,890,000						40,407,000		32,890,000
Prepayments and other current assets	43,316,000					60,441,000						43,316,000		60,441,000
Total current assets	1,194,638,000					1,167,168,000						1,194,638,000		1,167,168,000
Investments	109,424,000					103,661,000						109,424,000		103,661,000
Property, plant and equipment	7,646,222,000					7,218,503,000						7,646,222,000		7,218,503,000
Less accumulated depreciation, depletion and amortization	3,361,208,000					3,103,323,000						3,361,208,000		3,103,323,000		2,872,465,000
Net property, plant and equipment	4,285,014,000					4,115,180,000						4,285,014,000		4,115,180,000		3,894,117,000
Deferred charges and other assets: [Abstract]
Goodwill	634,931,000	[4]				634,633,000	[4]					634,931,000	[4]	634,633,000	[4]	629,463,000	[4]
Other	311,275,000					257,636,000						311,275,000		257,636,000
Total deferred charges and other assets	967,049,000					917,540,000						967,049,000		917,540,000
Total assets	6,556,125,000					6,303,549,000						6,556,125,000		6,303,549,000		5,990,952,000
Current liabilities:
Short-term borrowings	0					20,000,000						0		20,000,000
Long-term debt due within one year	139,267,000					72,797,000						139,267,000		72,797,000
Accounts payable	337,228,000					301,132,000						337,228,000		301,132,000
Taxes payable	70,176,000					56,186,000						70,176,000		56,186,000
Dividends payable	31,794,000					30,773,000						31,794,000		30,773,000
Accrued Compensation	47,804,000					40,121,000						47,804,000		40,121,000
Other accrued liabilities	259,320,000					222,639,000						259,320,000		222,639,000
Total current liabilities	898,753,000					768,076,000						898,753,000		768,076,000
Long-term debt	1,285,411,000					1,433,955,000						1,285,411,000		1,433,955,000
Deferred credits and other liabilities:
Deferred income taxes	769,166,000					672,269,000						769,166,000		672,269,000
Other liabilities	827,228,000					736,447,000						827,228,000		736,447,000
Total deferred credits and other liabilities	1,596,394,000					1,408,716,000						1,596,394,000		1,408,716,000
Stockholders' equity:
Preferred stocks	15,000,000					15,000,000						15,000,000		15,000,000
Common stockholders' equity:
Common stock shares authorized - 500,000,000 - issued - $1.00 par value, 189,332,485 shares at December 31, 2011 and 188,901,379 shares at December 31, 2010	189,332,000					188,901,000						189,332,000		188,901,000
Other paid-in capital	1,035,739,000					1,026,349,000						1,035,739,000		1,026,349,000
Retained earnings	1,586,123,000					1,497,439,000						1,586,123,000		1,497,439,000
Accumulated other comprehensive loss	(47,001,000)					(31,261,000)						(47,001,000)		(31,261,000)		(20,833,000)
Treasury stock at cost - 538,921 shares	(3,626,000)					(3,626,000)						(3,626,000)		(3,626,000)
Total common stockholders' equity	2,760,567,000					2,677,802,000						2,760,567,000		2,677,802,000		2,556,647,000		2,746,076,000
Total stockholders' equity	2,775,567,000					2,692,802,000						2,775,567,000		2,692,802,000
Total liabilities and stockholders' equity	6,556,125,000					6,303,549,000						6,556,125,000		6,303,549,000
Net cash provided by operating activities												626,648,000		551,633,000		846,686,000
Investing activities:
Capital expenditures												(497,000,000)		(449,282,000)		(448,675,000)
Net proceeds from sale or disposition of property and other												40,107,000		76,386,000		26,679,000
Investments												(10,302,000)		704,000		(3,740,000)
Net cash used in investing activities												(464,545,000)		(407,944,000)		(432,146,000)
Financing activities:
Issuance of short-term borrowings												0		20,000,000		10,300,000
Repayment of short-term borrowings												(20,000,000)		(10,300,000)		(105,100,000)
Issuance of long-term debt												300,000		20,200,000		145,000,000
Repayment of long-term debt												(85,151,000)		(13,668,000)		(292,907,000)
Proceeds from issuance of common stock												5,744,000		4,972,000		65,207,000
Dividends paid												(123,323,000)		(119,157,000)		(115,023,000)
Excess tax benefit on stock-based compensation												1,239,000		1,186,000		601,000
Net cash provided by (used in) financing activities												(221,191,000)		(96,767,000)		(291,922,000)
Increase (decrease) in cash and cash equivalents												(59,302,000)		46,960,000		123,400,000
Cash and cash equivalents - beginning of year					222,074,000						175,114,000	222,074,000		175,114,000		51,714,000
Cash and cash equivalents - end of year	162,772,000					222,074,000						162,772,000		222,074,000		175,114,000
MDU Resources Group, Inc [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues												518,268,000		503,658,000		514,519,000
Operating expenses												450,579,000		431,293,000		458,130,000
Operating income (loss)												67,689,000		72,365,000		56,389,000
Other income												2,710,000		5,734,000		6,588,000
Interest expense												18,660,000		16,664,000		13,996,000
Income before income taxes												51,739,000		61,435,000		48,981,000
Income taxes												10,476,000		17,983,000		13,279,000
Equity in earnings of subsidiaries												171,763,000		197,207,000		(158,976,000)
Net income (loss)												213,026,000		240,659,000		(123,274,000)
Dividends on preferred stocks												685,000		685,000		685,000
Earnings on common stock												212,341,000		239,974,000		(123,959,000)
Current assets:
Cash and cash equivalents	6,900,000					6,275,000						6,900,000		6,275,000		30,103,000
Receivables, net	67,761,000					76,757,000						67,761,000		76,757,000
Accounts receivable from subsidiaries	28,734,000					27,837,000						28,734,000		27,837,000
Inventories	42,596,000					34,583,000						42,596,000		34,583,000
Deferred income taxes	2,000					0						2,000		0
Prepayments and other current assets	12,154,000					15,473,000						12,154,000		15,473,000
Total current assets	158,147,000					160,925,000						158,147,000		160,925,000
Investments	47,835,000					48,038,000						47,835,000		48,038,000
Investment in subsidiaries	2,402,891,000					2,336,133,000						2,402,891,000		2,336,133,000
Property, plant and equipment	1,453,089,000					1,388,128,000						1,453,089,000		1,388,128,000
Less accumulated depreciation, depletion and amortization	605,510,000					583,447,000						605,510,000		583,447,000
Net property, plant and equipment	847,579,000					804,681,000						847,579,000		804,681,000
Deferred charges and other assets: [Abstract]
Goodwill	4,812,000					4,812,000						4,812,000		4,812,000
Other	166,732,000					119,081,000						166,732,000		119,081,000
Total deferred charges and other assets	171,544,000					123,893,000						171,544,000		123,893,000
Total assets	3,627,996,000					3,473,670,000						3,627,996,000		3,473,670,000
Current liabilities:
Short-term borrowings	0					20,000,000						0		20,000,000
Long-term debt due within one year	107,000					107,000						107,000		107,000
Accounts payable	37,986,000					36,235,000						37,986,000		36,235,000
Accounts payable to subsidiaries	4,868,000					9,445,000						4,868,000		9,445,000
Taxes payable	18,304,000					8,104,000						18,304,000		8,104,000
Deferred income taxes	0					469,000						0		469,000
Dividends payable	31,794,000					30,773,000						31,794,000		30,773,000
Accrued Compensation	10,173,000					11,540,000						10,173,000		11,540,000
Other accrued liabilities	27,064,000					26,002,000						27,064,000		26,002,000
Total current liabilities	130,296,000					142,675,000						130,296,000		142,675,000
Long-term debt	280,781,000					280,889,000						280,781,000		280,889,000
Deferred credits and other liabilities:
Deferred income taxes	137,751,000					103,725,000						137,751,000		103,725,000
Other liabilities	303,601,000					253,579,000						303,601,000		253,579,000
Total deferred credits and other liabilities	441,352,000					357,304,000						441,352,000		357,304,000
Stockholders' equity:
Preferred stocks	15,000,000					15,000,000						15,000,000		15,000,000
Common stockholders' equity:
Common stock shares authorized - 500,000,000 - issued - $1.00 par value, 189,332,485 shares at December 31, 2011 and 188,901,379 shares at December 31, 2010	189,332,000					188,901,000						189,332,000		188,901,000
Other paid-in capital	1,035,739,000					1,026,349,000						1,035,739,000		1,026,349,000
Retained earnings	1,586,123,000					1,497,439,000						1,586,123,000		1,497,439,000
Accumulated other comprehensive loss	(47,001,000)					(31,261,000)						(47,001,000)		(31,261,000)
Treasury stock at cost - 538,921 shares	(3,626,000)					(3,626,000)						(3,626,000)		(3,626,000)
Total common stockholders' equity	2,760,567,000					2,677,802,000						2,760,567,000		2,677,802,000
Total stockholders' equity	2,775,567,000					2,692,802,000						2,775,567,000		2,692,802,000
Total liabilities and stockholders' equity	3,627,996,000					3,473,670,000						3,627,996,000		3,473,670,000
Net cash provided by operating activities												217,514,000		185,887,000		209,128,000
Investing activities:
Capital expenditures												(74,580,000)		(114,045,000)		(120,352,000)
Net proceeds from sale or disposition of property and other												720,000		625,000		1,039,000
Investments in and advances to subsidiaries												(5,701,000)		(1,636,000)		0
Investments From and Advances From Subsidiaries												0		0		2,916,000
Disposition of investments in subsidiaries												0		0		20,000,000
Investments												0		(742,000)		(637,000)
Net cash used in investing activities												(79,561,000)		(115,798,000)		(97,034,000)
Financing activities:
Issuance of short-term borrowings												0		20,000,000		0
Repayment of short-term borrowings												(20,000,000)		0		0
Issuance of long-term debt												0		0		50,000,000
Repayment of long-term debt												(107,000)		(107,000)		(85,104,000)
Proceeds from issuance of common stock												5,744,000		4,972,000		65,207,000
Dividends paid												(123,323,000)		(119,157,000)		(115,023,000)
Excess tax benefit on stock-based compensation												358,000		375,000		264,000
Net cash provided by (used in) financing activities												(137,328,000)		(93,917,000)		(84,656,000)
Increase (decrease) in cash and cash equivalents												625,000		(23,828,000)		27,438,000
Cash and cash equivalents - beginning of year					6,275,000						30,103,000	6,275,000		30,103,000		2,665,000
Cash and cash equivalents - end of year	6,900,000					6,275,000						6,900,000		6,275,000		30,103,000
Debt Disclosure [Abstract]
Long-term Debt	280,900,000											280,900,000
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	100,000											100,000
2012	100,000											100,000
2013	100,000											100,000
2014	100,000											100,000
2015	100,000											100,000
2016	50,000,000											50,000,000
Thereafter 2016	230,500,000											230,500,000
Cash Dividends Paid to Parent Company [Abstract]
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries												$ 96,100,000		$ 96,400,000		$ 116,300,000

[1] 2010 reflects a natural gas gathering arbitration charge of $16.5 million (after tax). For more information, see Note 19.

[2] 2011 reflects an arbitration charge of $13.0 million (after tax) related to a guarantee of a construction contract. For more information, see Note 19.

[3] 2010 reflects a $13.8 million (after tax) gain on the sale of the Brazilian Transmission Lines. For more information, see Note 4.

[4] Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.